Photodetector, optical communication device equipped with the same, method for making of photodetector, and method for making of optical communication device

ABSTRACT

The present invention provides a photodetector which solves the problem of low sensitivity of a photodetector, an optical communication device equipped with the same, and a method for making the photodetector, and a method for making the optical communication device. The photodetector includes a substrate, a lower cladding layer arranged on the substrate, an optical waveguide arranged on the lower cladding layer, an intermediate layer arranged on the optical waveguide, a optical absorption layer arranged on the intermediate layer, a pair of electrodes arranged on the optical absorption layer, and wherein the optical absorption layer includes a IV-group or III-V-group single-crystal semiconductor, and the optical absorption layer absorbs an optical signal propagating through the optical waveguide.

TECHNICAL FIELD

The present invention relates to a photodetector and an opticalcommunication device equipped with the same, and a method for making thephotodetector and a method for making the optical communication device.

BACKGROUND ART

In recent years, it is expected that it is possible to realize a highperformance and cheap optical interconnect device by manufacturing anoptical integrated circuit using the process technology for a siliconComplementary Metal Oxide Semiconductor (CMOS) circuit. It is aphotodetector that is in one of devices indispensable to the opticalintegrated circuit. The photodetector is desirable to be high-speed andsensitive, with short optical absorption length and small in size. Froma point of view of the cost and yield, a photodiode using Si_(x)Ge_(1-x)(0≦x≦1) which can be monolithically integrated with a Si waveguide isconsidered promising.

For example, a waveguide detector using poly germanium stacked on asilicon on Insulator (SOI) substrate is disclosed in the patentliterature 1.

Also, the nonpatent literature 1 discloses a process for forming a Gelayer having low defect density by performing selective epitaxial growthof a Ge layer in a trench part of SiO₂ film using a Si layer as anunderlayer, and further performing lateral overgrowth of the Ge layer onthe SiO₂ film.

-   Patent Literature 1: Japanese Patent Laid-Open Application    Publication (Translation of PCT Application) No. 2006-522465    (paragraphs [0030], [0018], FIG. 5)-   NonPatent Literature 1: J. S. Park et al., Low-Defect-Density Ge    Epitaxy on Si(001) Using Aspect Ratio Trapping and Epitaxial Lateral    Overgrowth, Electrochemical and Solid-State Letters 12 p. 142 (2009)    (FIG. 1).

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

In the photodetector disclosed in patent literature 1, the defectdensity of the optical absorption layer is high because poly germaniumused as the optical absorption layer has a polycrystalline structure.Therefore, there has been a problem that a dark current flowing betweenelectrodes is large and the sensitivity of the photodetector is low.

Further, although the method for forming a Ge layer with low defectdensity is disclosed in nonpatent literature 1, it is not disclosed toapply this method to the process for making the photodetectors.

The object of the present invention is to provide a photodetector whichsolves the above mentioned problem of low sensitivity of photodetector,an optical communication device equipped with that, and a method formaking the photodetector and a method for making the opticalcommunication device.

Means for Solving a Problem

A photodetector of the present invention includes a substrate, a lowercladding layer arranged on the substrate, an optical waveguide arrangedon the lower cladding layer, an intermediate layer arranged on theoptical waveguide, a optical absorption layer arranged on theintermediate layer and a pair of electrodes arranged on the opticalabsorption layer, and wherein the optical absorption layer comprises aIV-group or III-V-group single-crystal semiconductor, and the opticalabsorption layer absorbs an optical signal propagating through theoptical waveguide.

Also, a optical communication device of the present invention includes aphotodetector, and wherein the photodetector includes a substrate, alower cladding layer arranged on the substrate, an optical waveguidearranged on the lower cladding layer, an intermediate layer arranged onthe optical waveguide, a optical absorption layer arranged on theintermediate layer and a pair of electrodes arranged on the opticalabsorption layer, and wherein the optical absorption layer comprises aIV-group or III-V-group single-crystal semiconductor, and the opticalabsorption layer absorbs an optical signal propagating through theoptical waveguide.

Further, a method for making a photodetector of the present inventionincludes, making a crystal grow on a growth underlayer in a firstelement which includes a substrate, a lower cladding layer arranged onthe substrate, an optical waveguide arranged on the lower claddinglayer, an intermediate layer arranged on the optical waveguide, and thegrowth underlayer arranged on the substrate, to a direction nearlyvertical to the growth underlayer, after the grown crystal havingreached the height of the intermediate layer, forming a opticalabsorption layer on the intermediate layer by making a crystal grow to adirection nearly parallel to a surface of the intermediate layer, andforming a pair of electrodes on the optical absorption layer.

Further, a method for making an optical communication device of thepresent invention includes, forming an optical communication deviceusing a photodetector made by making a crystal grow on a growthunderlayer in a first element which includes a substrate, a lowercladding layer arranged on the substrate, an optical waveguide arrangedon the lower cladding layer, an intermediate layer arranged on theoptical waveguide, and the growth underlayer arranged on the substrate,to a direction nearly vertical to the growth underlayer, after the growncrystal having reached the height of the intermediate layer, forming aoptical absorption layer on the intermediate layer by making a crystalgrow to a direction nearly parallel to a surface of the intermediatelayer, and forming a pair of electrodes on the optical absorption layer.

Effect of the Invention

According to the photodetector of the present invention, thephotodetector can be made sensitive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are (A) a plan view of a photodetector according to a firstexemplary embodiment of the present invention, (B) a cross-section viewtaken along the line 1B-1B in the plan view (A), and (C) a cross-sectionview taken along the line 1C-1C in the plan view (A).

FIG. 2 is a cross-section view of another photodetector according to thefirst exemplary embodiment of the present invention.

FIGS. 3AI-3DI and FIGS. 3AII-3DII are figures showing a method formaking the photodetector according to the first exemplary embodiment ofthe present invention, and are (AI) a cross-section view taken along theline 3AI-3AI in a plan view (AII) of an element in a prescribed processand (AII) a plan view, (BI) a cross-section view taken along the line3BI-3BI in a plan view (BII) of the element in another process and (BII)a plan view, (CI) a cross-section view taken along the line 3CI-3CI in aplan view (CII) in further another process and (CII) a plan view, and(DI) a cross-section view taken along the line 3DI-3DI in a plan view(DII) in further another process and (DII) a plan view.

FIGS. 4AI-4DI and FIGS. 4AII-4DII are figures showing a method formaking the photodetector according to the first exemplary embodiment ofthe present invention, and are (AI) a cross-section view taken along theline 4AI-4AI in a plan view (AII) of an element in a prescribed processand (AII) a plan view, (BI) cross-section view taken along the line4BI-4BI in a plan view (BII) of the element in another process and (BII)a plan view, (CI) a cross-section view taken along the line 4CI-4CI in aplan view (CII) in further another process and (CII) a plan view, and(DI) a cross-section view taken along the line 4DI-4DI in a plan view(DII) in further another process and (DII) a plan view.

FIGS. 5A-5C are (A) a plan view of a photodetector according to a secondexemplary embodiment of the present invention, (B) a cross-section viewtaken along the line 5B-5B in the plan view (A), and (C) a cross-sectionview taken along the line 5C-5C in the plan view (A).

FIGS. 6AI-BI and FIGS. 6AII-6BII are figures showing a method for makingthe photodetector according to the second exemplary embodiment of thepresent invention, and are (AI) a cross-section view taken along theline 6AI-6AI in a plan view (AII) of an element in a prescribed processand (AII) a plan view, and (BI) a cross-section view taken along theline 6BI-6BI in a plan view (BII) of the element in another process and(BII) a plan view.

FIG. 7A and FIG. 7B are (A) a plan view of a photodetector according toa third exemplary embodiment of the present invention and (B) across-section view taken along the line 7B-7B in the plan view (A).

FIG. 8 is a cross-section view of a photodetector according to a fourthexemplary embodiment of the present invention.

FIG. 9 is a cross-section view of a photodetector according to a fifthexemplary embodiment of the present invention.

FIG. 10 is a cross-section view of an optical communication deviceaccording to a sixth exemplary embodiment of the present invention.

FIG. 11 is a cross-section view of an optical communication deviceaccording to a seventh exemplary embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Next, the exemplary embodiments of the present invention will bedescribed in detail with reference to drawings.

First Exemplary Embodiment

FIG. 1(A) is a plan view of a photodetector 100 according to a firstexemplary embodiment of the present invention. FIG. 1(B) is across-section view taken along the line 1B-1B in the plan view of FIG.1(A). FIG. 1(C) is a cross-section view taken along the line 1C-1C inthe plan view of FIG. 1(A). Further, for convenience of explanation, anupper cladding layer 107 is not indicated in the figure of FIG. 1(A). Asshown in FIGS. 1(B), (C), the photodetector 100 is provided with asubstrate 104 made of Si and the like, a lower cladding layer 105arranged on the substrate 104, and an optical waveguide 101 arranged onthe lower cladding layer 105. As for the lower cladding layer 105, forexample, SiO₂ can be used. Further, as for the optical waveguide 101,for example, Si can be used. Further, an intermediate layer 106 made ofSiO₂ and the like is arranged on the optical waveguide 101. Further, aoptical absorption layer 102 is arranged on the intermediate layer 106.Further, a pair of electrodes 103 is arranged on the optical absorptionlayer 102. The optical absorption layer 102 is made of single-crystalSi_(x)Ge_(1-x) (0≦x≦1). The optical absorption layer 102 absorbs anoptical signal that propagates through the optical waveguide 101. Thephotodetector 100 has an upper cladding layer 107 which covers theelectrodes 103 and the optical absorption layer 102.

In the photodetector 100 of this exemplary embodiment, the opticalabsorption layer 102 is made of single-crystal Si_(x)Ge_(1-x) (0≦x≦1).Therefore, the defect density of the optical absorption layer 102provided for the photodetector 100 of this exemplary embodiment is lowerthan the defect density of the optical absorption layer which is formedby depositing polycrystalline germanium described in patentliterature 1. As a result, in the photodetector 100 of this exemplaryembodiment, it is possible to reduce a dark current arising between theelectrodes 103 and to prevent the sensitivity of the photodetector 100from decreasing. That is, the photodetector 100 can be made sensitive.

Further, according to the photodetector 100 of this exemplaryembodiment, an impurity doped region becomes unnecessary at the portionwhere the optical waveguide 101 and the optical absorption layer 102 areoptically connected. Therefore, it is possible to prevent an opticalsignal propagating through the optical waveguide 101 from being absorbedby impurities before reaching the optical absorption layer 102.

Also, the photodetector 100 according to this exemplary embodiment canachieve a strong optical confinement effect because the photodetector100 is provided with the intermediate layer 106 made of SiO₂, whoserefractive index is relatively small, under the optical absorption layer102. As a result, the photodetector 100 can operate at high speed.Further, it is possible to shorten the absorption length of the lightwhich is absorbed by the optical absorption layer.

In the above-mentioned description, although the materials of thesubstrate 104, the optical waveguide 101, and the optical absorptionlayer 102 are specified, each composition of the photodetector 100according to this exemplary embodiment is not limited to thesematerials. The material used for each composition can be selectedappropriately according to the purpose and the wavelength used, and forexample, it is possible to use a IV-group semiconductor such as Si, Ge,C and Sn, or III-V-group semiconductor. Preferably in particular, it isdesirable to use Si_(x)Ge_(1-x) (0≦x≦1), which can absorb the light withthe optical communication wavelength band, for the optical absorptionlayer 102 with using the Si substrate 104 for which the standard siliconCMOS process can be used.

A bulk Si substrate may be used for the substrate 104, and an SOIsubstrate may also be used. For the lower cladding layer 105, theintermediate layer 106, and the upper cladding layer 107, it isdesirable to use SiO₂, SiON, SiN and the like which have a high affinityfor Si and Si_(x)Ge_(1-x), and in particular, it is desirable to useSiO₂ which can easily form a film with the stable state.

The composition ratio of Si and Ge which compose the optical absorptionlayer 102 may be determined appropriately in view of the wavelength ofan incident light and deposition conditions. At that time, it isdesirable to determine the composition ratio of Si and Ge which composethe optical absorption layer 102 so that the optical absorption layer102 may have enough absorption coefficient to the incident lightwavelength.

By controlling the composition ratio of Si and Ge which compose theoptical absorption layer 102, it is possible to shorten the absorptionlength of the light. For example, if the optical communicationwavelength band is used, it is possible to shorten the absorption lengthof the light by composing the optical absorption layer 102 using Ge onlysetting x at 0 in Si_(x)Ge_(1-x) (0≦x≦1).

It is desirable that the thickness of the intermediate layer 106 shouldbe thinner, and in particular, it is desirable to make the thickness ofthe intermediate layer 106 almost equal to or thinner than that of theoptical waveguide 101. As a result, it is possible to make an opticalsignal, which has propagated through the optical waveguide 101, shift tothe optical absorption layer 102 with a short coupling length.

Further, in order to realize high-speed operation of the photodetector,it is desirable that an interval of a pair of electrodes 103 should besmaller, and more specifically, it is desirable to make it equal to orsmaller than 1 μm.

The composition without the upper cladding layer 107 is also availablefor the photodetector 100. The upper cladding layer 107 has not only thefunction as a cladding but also the function as a protection film.

It is acceptable to remove the upper cladding layer 107 on theelectrodes 103 by etching and the like. As a result, it is possible toconnect electrically the electrodes 103 to an external circuit.

The photodetector 100 according to this exemplary embodiment may be anMSM (Metal-Semiconductor-Metal) type photodiode, and may also be a PIN(P-type-Intrinsic-N-type) type photodiode. In a case where thephotodetector 100 is the MSM type photodiode, for the electrodes 103, itis desirable to use the metal which is used in the standard silicon CMOSprocess and forms high-quality Schottky junction with a semiconductor.Further, high-quality a plurality of metal materials, such as barriermetals and the metal to improve the adhesion to a semiconductor surface,are also available for the electrode 103. The photodetector 100 shown inFIG. 1 is a photodiode with an MSM structure in a lateral direction(horizontal direction) which is composed of one of the electrodes 103,the optical absorption layer 102, and the other of the electrodes 103.

An example of the case where the photodetector which is a PIN typephotodiode will be described using FIG. 2. FIG. 2 is a cross-sectionview of the photodetector 150. The photodetector 150 has a PIN structureof the lateral direction (horizontal direction). The photodetector 150is provided with the substrate 104, the lower cladding layer 105arranged on the substrate 104, and the optical waveguide 101 arranged onthe lower cladding layer 105. Further, the intermediate layer 106 isarranged on the optical waveguide 101. Further, the optical absorptionlayer 102 is arranged on the intermediate layer 106. Further, a p-typesemiconductor 108 and an n-type semiconductor 109 are arranged on theoptical absorption layer 102. Also, a p-electrode 110 is arranged on thep-type semiconductor 108, and an n-electrode 111 is arranged on then-type semiconductor 109. Although it is possible to use Ge and the likeas the material for the p-type semiconductor 108 and the n-typesemiconductor 109, the material is not limited to those.

In the process for producing the PIN type photodiode, because ofperforming ion implantation in the neighborhood of the p-electrode 110and the n-electrode 111, the process is more complicated than that forproducing the MSM type photodiode. However, the PIN type photodiode canmake a dark current smaller than that of the MSM type photodiode, and itcan realize more stable operation.

The photodetector 100 according to this exemplary embodiment can be usedas an optical receiver of an optical communication device. For example,by forming the photodetector of the present invention on a substrate,and further integrating an LSI electronic circuit on the same substratemonolithically, an optical interconnection module can be composed.

Next, a method for making the photodetector according to this exemplaryembodiment will be described using FIG. 3 and FIG. 4. FIG. 3(AI)-FIG.3(DI) and FIG. 4(AI)-FIG. 4(DI) are cross-section views of thephotodetector in each production process. FIG. 3(AII)-FIG. 3(DII) andFIG. 4(AII)-FIG. 4(DII) are plan views of the photodetector in eachproduction process. For convenience of explanation, the upper claddinglayer 107 is not indicated in FIG. 3(AII)-FIG. 3(DII) and FIG.4(AII)-FIG. 4(DII).

First, the lower cladding layer 105 is formed on the substrate 104, andthe semiconductor layer 115 is formed on the formed lower cladding layer105 (FIG. 3(AI) and FIG. 3(AII)). Further, an SOI substrate having suchstructure may be used.

Next, an optical waveguide 101 and a growth underlayer 113 are formed byetching the semiconductor layer 115 (FIG. 3(BI)). At this time, it ispreferable to etch the semiconductor layer 115 so that the growthunderlayer 113 may be arranged only in one area of edge sides of theoptical waveguide 101 on the substrate (FIG. 3(BII)).

Next, the intermediate layer 106 is formed using the chemical vapordeposition method and the like (FIG. 3(CI) and FIG. 3(CII)).

Next, the intermediate layer 106 is planarized using the ChemicalMechanical Polishing (CMP) and the like (FIG. 3(DI) and FIG. 3(DII)).

Next, the intermediate layer 106 on the area of the growth underlayer113 is removed by etching and the like (FIG. 4(AI) and FIG. 4(AII)). Anelement in the state shown in FIG. 4(AI) and FIG. 4(AII) is called afirst element.

Next, epitaxial growth of the growth underlayer 113 is performed usingthe chemical vapor deposition method and the like with GeH₄ and the likeas source gas. As a result, first, on the growth underlayer 113, alongitudinal growth semiconductor layer 114 of a crystal grown in alongitudinal direction is formed. Here, the longitudinal direction meansa direction nearly vertical to the growth underlayer 113. The crystalkeeps growing even if the height of a growth front of the longitudinalgrowth semiconductor layer 114 becomes equal to the height of theintermediate layer 106. Then, the crystal also grows into the lateraldirection of the longitudinal growth semiconductor layer 114. And, bythe crystal grown into the lateral direction, the optical absorptionlayer 102 is formed on the intermediate layer 106 (FIG. 4(BI) and FIG.4(BII)). Here, the lateral direction means a direction nearly parallelto a top surface of the intermediate layer 106. Also, it will be calledlongitudinal growth that the crystal grows into a longitudinaldirection. And, it will be called lateral overgrowth that the crystalgrows into a longitudinal direction and further grows into a lateraldirection.

Next, the growth underlayer 113 and the longitudinal growthsemiconductor layer 114 formed on the area of the growth underlayer areremoved by etching and the like (FIG. 4(CI) and FIG. 4(CII)).

Finally, the electrodes 103 are formed on the optical absorption layer102, and further the upper cladding layers 107 is formed (FIG. 4(DI) andFIG. 4(DII)).

According to the method for making the photodetector 100 of thisexemplary embodiment, by forming the optical absorption layer 102 usingthe lateral overgrowth, the defect density in the optical absorptionlayer 102 can be made smaller. This is because by using the lateralovergrowth, it is possible to make the optical absorption layer 102formed as a single crystal structure. As a result, it is possible tosuppress a dark current arising between the electrodes 103 and toproduce the high-sensitive photodetector 100.

Also, by forming the optical absorption layer 102 using the lateralovergrowth, easily, it is possible to make the optical absorption layer102 optically connected to the optical waveguide 101 with a high degreeof accuracy by self alignment. In the photodetector 100 produced in thisway, by the evanescent coupling, an optical signal is transmitted fromthe waveguide 101 to the optical absorption layer 102 high-efficiently.

Further, by forming the optical absorption layer 102 using the lateralovergrowth, the defect density in the optical absorption layer 102 canbe made smaller than the defect density of the optical absorption layerformed by using the longitudinal growth. This is because, for example,that if a crystal of SiGe is made to grow on the growth underlayer 113made of Si, a lot of threading dislocations caused by the difference inthe lattice constant between Si and SiGe are included in SiGe obtainedby the longitudinal growth, on the other hand, few of the threadingdislocations are included in SiGe obtained by the lateral overgrowth.

The intermediate layer 106 functions as a mask layer when forming thelongitudinal growth semiconductor layer 114 using the chemical vapordeposition method and the like. Thus, by masking the optical waveguide101 with the intermediate layer 106, on the upper part of the opticalwaveguide 101, without making Si_(x)Ge_(1-x) (0≦x≦1) grow into thelongitudinal direction, it is possible to form the optical absorptionlayer 102, which has grown from the longitudinal growth semiconductorlayer 114 into the lateral direction, on the intermediate layer.

Further, according to the method for making the photodetector 100 ofthis exemplary embodiment, the growth underlayer 113 and thelongitudinal growth semiconductor layer 114 are removed by etching andthe like. Therefore, the photodetector 100 does not include the growthunderlayer 113 and the longitudinal growth semiconductor layer 114, andwhen using the photodetector 100, there is no case that an opticalsignal propagating through the optical waveguide 101 leaks out to thegrowth underlayer 113. Also, there is no case that the optical signalpropagating through the optical waveguide 101 is absorbed by thelongitudinal growth semiconductor layer 114. Therefore, the sensitivityof the photodetector 100 can be made higher.

Further, in the above-mentioned description, although the opticalwaveguide 101 and the growth underlayer 113 are formed from the samesemiconductor layer 116, it is possible to form each independently. Byforming the optical waveguide 101 and the growth underlayer 113 from thesame semiconductor layer 116, the production process can be made simple.

The semiconductor material used for the growth underlayer 113 is notlimited in particular, and it is also possible to use III-V-groupsemiconductors or IV-group semiconductors such as Si, Ge, C, Sn and thelike.

As the semiconductor material used for the growth underlayer 113, forexample, single crystal Si can be used. If single crystal Si is used forthe growth underlayer 113, it is possible to obtain Si_(x)Ge_(1-x)(0≦x≦1) of high-quality single crystal structure. For example, it ispossible to obtain single crystal Si and/or single crystal Ge bychanging the composition ratio of the grown crystal through changing theratio of the gas to be used at the time of growth.

In the above-mentioned description, although it has described about theprocess for producing the photodetector 100 in which the opticalabsorption layer 102 is composed of a single crystal structure using thesemiconductor material having a single crystal structure as the growthunderlayer 113, the growth underlayer 113 and the optical absorptionlayer 102 are not limited to a single crystal structure. For example, itis also possible to form the optical absorption layer with apolycrystalline structure or an amorphous structure usingpolycrystalline Si or amorphous Si as the growth underlayer. Also, inthis case, by forming the optical absorption layer with apolycrystalline structure or an amorphous structure using the lateralovergrowth, the defect density in the optical absorption layer can bemade smaller than that of the optical absorption layer with apolycrystalline structure or an amorphous structure formed using thelongitudinal growth. This is because, for example, that if a crystal ofSiGe is made to grow on the growth underlayer 113 made of Si, a lot ofthreading dislocations caused by the difference in the lattice constantbetween Si and SiGe are included in SiGe obtained by the longitudinalgrowth, on the other hand, few of the threading dislocations areincluded in SiGe obtained by the lateral overgrowth. Because a darkcurrent arising between the electrodes can be suppressed by reducing thedefect density in the optical absorption layer, it is possible toproduce a high-sensitivity photodetector by the method for making thephotodetector according to this exemplary embodiment.

Second Exemplary Embodiment

Next, a photodetector 200 of a second exemplary embodiment of thepresent invention will be described. FIG. 5(A) is a plan view of thephotodetector 200 according to the second exemplary embodiment of thepresent invention. FIG. 5(B) is a cross-section view taken along theline 5B-5B in the plan view of FIG. 5(A). For convenience ofexplanation, an upper cladding layer 207 is not indicated in the figureof FIG. 5(A). FIG. 5(C) is a cross-section view taken along the line5C-5C in the plan view of FIG. 5(A). As shown in FIG. 5(B), FIG. 5(C),the photodetector 200 is provided with a substrate 204, a lower claddinglayer 205 arranged on the substrate 204, and an optical waveguide 201arranged on the lower cladding layer 205. Further, an intermediate layer206 is arranged on the optical waveguide 201. Further, an opticalabsorption layer 202 is arranged on the intermediate layer 206. Further,a pair of electrodes 203 is arranged on the optical absorption layer202. The optical absorption layer 202 is made of a IV-group orIII-V-group single-crystal semiconductor. The optical absorption layer202 absorbs an optical signal propagating through the optical waveguide201. The photodetector 200 includes an upper cladding layer 207 whichcovers the electrodes 203 and the optical absorption layer 202.

Further, the photodetector 200 is provided with a growth underlayer 213arranged on the lower cladding layer 205 and a longitudinal growthsemiconductor layer 214 arranged on the growth underlayer 213.

According to the photodetector 200 of this exemplary embodiment, theoptical absorption layer 202 is made of a IV-group or III-V-groupsingle-crystal semiconductor. Therefore, the defect density of theoptical absorption layer 202 provided for the photodetector 200 of thisexemplary embodiment is lower. As a result, in the photodetector 200 ofthis exemplary embodiment, it is possible to reduce a dark currentarising between the electrodes 203, and to prevent the sensitivity ofthe photodetector 200 from decreasing.

Next, a method for making the photodetector according to this exemplaryembodiment will be described using FIG. 6.

FIG. 6(AI)-FIG. 6(BI) are cross-section views of the photodetector ineach production process. FIG. 6(AII)-FIG. 6(BII) are plan views of thephotodetector in each production process. For convenience ofexplanation, the upper cladding layer 207 is not indicated in thefigures of FIG. 6(AII)-FIG. 6(BII).

First, the photodetector shown in FIG. 6(AI), FIG. 6(AII) is prepared.Because the process for producing this photodetector is the same as theprocess for producing the photodetector described using FIG. 3(AI)-FIG.3(DI) and FIG. 4(AI)-FIG. 4(BI) in the first exemplary embodiment, itsdescription will be omitted.

Next, the electrodes 203 are formed on the optical absorption layer 202,and further the upper cladding layers 207 are formed (FIG. 6(BI), FIG.6(BII)).

In the process for producing the photodetector according to thisexemplary embodiment, the number of production process is smallercompared with the method for making the photodetector according to thefirst exemplary embodiment. This is because while removing the growthunderlayer 113 and the longitudinal growth semiconductor layer 114 byetching and the like in the method for making the photodetectoraccording to the first exemplary embodiment (FIG. 4(CI)), this removalprocess is omitted in this exemplary embodiment. In other words, in theproduction process of this exemplary embodiment, the growth underlayer213 and the longitudinal growth semiconductor layer 214 are not removedby etching and the like. Therefore, in the method for making thephotodetector according to this exemplary embodiment, the number ofproduction process decreases, and it is possible to produce thephotodetector with lower cost using this production process.

Although the above-mentioned description has shown the method for makingthe photodetector 100 in which the optical absorption layer 102 has asingle crystal structure and semiconductor material of a single crystalstructure is used as the growth underlayer 113, the growth underlayer113 and the optical absorption layer 102 are not limited to a singlecrystal structure. That is, it is possible to obtain the opticalabsorption layer of a polycrystalline structure or an amorphousstructure using a IV-group or III-V-group semiconductor having apolycrystalline structure or an amorphous structure as the growthunderlayer 213.

Third Exemplary Embodiment

Next, a third exemplary embodiment of the present invention will bedescribed using FIG. 7. FIG. 7(A) is a plan view of a photodetector 300according to the third exemplary embodiment of the present invention.FIG. 7(B) is a cross-section view taken along the line 7B-7B in the planview of FIG. 7(A). For convenience of explanation, the upper claddinglayer 307 is not indicated in the figure of FIG. 7(A). As shown in FIGS.7(A) and 7(B), the photodetector 300 is provided with a substrate 304, alower cladding layer 305 arranged on the substrate 304, and an opticalwaveguide 301 arranged on the lower cladding layer 305. Further, anintermediate layer 306 is arranged on the optical waveguide 301.Further, an optical absorption layer 302 is arranged on the intermediatelayer 306. Further, a pair of electrodes 303 is arranged on the opticalabsorption layer 302. The optical absorption layer 302 is made of aIV-group or III-V-group semiconductor having a single crystal structure.The optical absorption layer 302 absorbs an optical signal 316propagating through the optical waveguide 301. The photodetector 300includes the upper cladding layer 307 which covers the electrodes 303and the optical absorption layer 302.

Further, as shown in FIG. 7(B), only an edge portion of the opticalabsorption layer 302 is arranged on the optical waveguide 301interleaved with the intermediate layer 306.

According to the photodetector 300 according to this exemplaryembodiment, because only an edge portion of the optical absorption layer302 is arranged on the optical waveguide 301 interleaved with theintermediate layer 306, it is possible to prevent an optical signal 316,which has transferred from the optical waveguide 301 to the opticalabsorption layer 302, from returning to the optical waveguide 301, andthe optical absorption length can be made shorter.

Fourth Exemplary Embodiment

Next, a fourth exemplary embodiment of the present invention will bedescribed using FIG. 8. A plan view of a photodetector 400 according tothe fourth exemplary embodiment of the present invention is shown inFIG. 8. The photodetector 400 is provided with, just like thephotodetector according to the first exemplary embodiment, a substrate,a lower cladding layer, an intermediate layer, an optical waveguide, aoptical absorption layer, a pair of electrodes, and an upper claddinglayer. For convenience of explanation, the upper cladding layer, theintermediate layer, the optical absorption layer, and the electrodes arenot indicated in the figure of FIG. 8.

The photodetector 400 has a taper structure 417 at an edge portion ofthe optical waveguide 401, and this taper structure 417 is arrangedunder the optical absorption layer interleaved with the intermediatelayer. For this reason, it is possible to reduce the reflection and thescattering of the light at an optical coupling part of the opticalwaveguide 401 and the optical absorption layer, and the sensitivity ofthe photodetector can be made higher.

Although in the example shown in FIG. 8, the optical waveguide 401having the taper structure 417 is arranged over a span of one end of thesubstrate to the other end, it is also available to arrange only an edgeportion of the taper structure 417 under the optical absorption layerjust like the third exemplary embodiment in which the length of theoptical waveguide 401 is shortened against the distance between one endof the substrate and the other end.

Fifth Exemplary Embodiment

Next, a fifth exemplary embodiment of the present invention will bedescribed using FIG. 9. FIG. 9 is a cross-section view of aphotodetector 500 according to the fifth exemplary embodiment of thepresent invention. The photodetector 500 is provided with, just like thephotodetector according to the first exemplary embodiment, a substrate504, a lower cladding layer 505, an intermediate layer 506, an opticalwaveguide 501, a pair of electrodes 503, an optical absorption layer,and an upper cladding layer 507.

Further, an Si_(y)Ge_(1-y) (0≦y≦1, y≠x) layer 518 is arranged on theoptical absorption layer 502. The Si_(y)Ge_(1-y) (0≦y≦1, y≠x) layer 518has the composition in which the light propagating through the opticalabsorption layer 502 is not absorbed due, to its wavelength. Thus, byarranging the Si_(y)Ge_(1-y) (0≦y≦1, y≠x) layer 518 on the opticalabsorption layer 502, it is possible to reduce the proportion of thelight which is absorbed by the electrodes 503, and the sensitivity ofthe photodetector can be made higher.

Because the Si_(y)Ge_(1-y) (0≦y≦1, y≠x) layer 518 may be made of thematerial which has a larger refractive index than that of the uppercladding layer, for example, a SiN layer may be used instead of theSi_(y)Ge_(1-y) (0≦y≦1, y≠x) layer 518. However, since the Si_(y)Ge_(1-y)(0≦y≦1, y≠x) layer 518 has the larger refractive index, it is preferableto use the Si_(y)Ge_(1-y) (0≦y≦1, y≠x) layer.

Sixth Exemplary Embodiment

Next, a sixth exemplary embodiment of the present invention will bedescribed using FIG. 10. FIG. 10 is a cross-section view of an opticalcommunication device 627 for 40 Gbps transmission equipped with aphotodetector 600. The photodetector 600 is mounted on a chip carrier623 arranged in the module case 626. The photodetector 600 is opticallyconnected to an optic fiber 624 and a lens 620. Further, thephotodetector 600 is electrically connected to a pre-amplifier ICthrough an electric wiring 621.

According to this exemplary embodiment, since the optical communicationdevice 627 is equipped with the high-sensitive photodetector 600, it ispossible to improve the performance of the optical communication device627.

Seventh Exemplary Embodiment

Next, a seventh exemplary embodiment of the present invention will bedescribed using FIG. 11. FIG. 11 is a cross-section view of an opticalcommunication device equipped with a photodetector 700. This opticalcommunication device is an inter-chip connection module 728. Thephotodetector 700 is irradiated by a concave mirror 736 with an opticalsignal from an optical signal input fiber 739 mounted on the firstmounting board 735. After that, the optical signal is transmittedthrough an electric wiring layer for photodiode 738. And, an electriccurrent depending on the optical signal flows into an LSI mounted on anLSI package 729. The electric wiring layer for photodiode 738 iselectrically connected to an electric wiring via for photodiode 732 inthe LSI package 729. Here, instead of the optical fiber for inputtingthe optical signal, it is also possible to use other method which isgenerally known such as a planar optical waveguide. Further, instead ofthe concave mirror 736, it is also possible to use a light collectionstructure such as a convex lens. Further, it is also possible to arrangea pre-amplifier for amplifying electric signal in the electric wiringlayer for photodiode 738 which is located just behind the photodiode.

An electric signal from LSI passes from an electric wiring via for alight source and modulation 730 through an electric wiring layer for alight source and modulation 733 mounted on the second mounting board737, and is converted into an optical signal by a VCSEL (Vertical CavitySurface Emitting Laser; surface emission-type laser) light source 731equipped with an electric modulation system. This optical signal isreflected by the concave mirror 736, and is sent to an optical signaloutput fiber 734. Instead of the VCSEL light source 731 equipped with anelectric modulation system, it is also possible to use other systemwhich electrically modulates an optical signal, for example, aMach-Zehnder type modulator which modulates an optical signal from anexternal light source by using electro-optical effect or thermo-opticeffect.

According to this exemplary embodiment, since the inter-chip connectionmodule 728 is equipped with the high-sensitive photodetector 700, it ispossible to improve the performance of the inter-chip connection module728.

The present invention is not limited to the above-mentioned exemplaryembodiments, can be variously modified within the scope of the inventiondescribed in the claims. It goes without saying that these modificationsare also included in the scope of the invention.

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2010-161707, filed on Jul. 16, 2010, thedisclosure of which is incorporated herein in its entirety by reference.

The whole or part of the exemplary embodiments disclosed above can bedescribed as, but not limited to, the following supplementary notes.

(Supplementary note 1) A photodetector comprising a substrate, a lowercladding layer arranged on the substrate, an optical waveguide arrangedon the lower cladding layer, an intermediate layer arranged on theoptical waveguide, a optical absorption layer arranged on theintermediate layer, a pair of electrodes arranged on the opticalabsorption layer, and wherein the optical absorption layer comprises aIV-group or III-V-group single-crystal semiconductor, and the opticalabsorption layer absorbs an optical signal propagating through theoptical waveguide.(Supplementary note 2) The photodetector according to supplementary note1, wherein the optical absorption layer comprises Si_(x)Ge_(1-x)(0≦x≦1).(Supplementary note 3) The photodetector according to supplementary note1 or 2, wherein only an edge portion of the optical absorption layer isarranged on the optical waveguide interleaved with the intermediatelayer.(Supplementary note 4) The photodetector according to any one ofsupplementary notes 1, 2 and 3, wherein an edge portion of the opticalwaveguide has a taper structure, and the edge portion is arranged underthe optical absorption layer interleaved with the intermediate layer.(Supplementary note 5) The photodetector according to any one ofsupplementary notes 1, 2, 3 and 4, wherein a Si_(y)Ge_(1-y) (0≦y≦1, y≠x)layer is arranged on the optical absorption layer.(Supplementary note 6) The photodetector according to any one ofsupplementary notes 1, 2, 3, 4 and 5, wherein the optical waveguide ismade of Si.(Supplementary note 7) The photodetector according to any one ofsupplementary notes 1, 2, 3, 4, 5 and 6, wherein the photodetector is anMSM type photodiode or a PIN type photodiode.(Supplementary note 8) An optical communication device, comprising thephotodetector according to any one of supplementary notes 1, 2, 3, 4, 5,6 and 7.(Supplementary note 9) A method for making a photodetector comprising,making a crystal grow on a growth underlayer in a first element whichcomprises a substrate, a lower cladding layer arranged on the substrate,an optical waveguide arranged on the lower cladding layer, anintermediate layer arranged on the optical waveguide, and the growthunderlayer arranged on the substrate, to a direction nearly vertical tothe growth underlayer, after the grown crystal having reached the heightof the intermediate layer, forming a optical absorption layer on theintermediate layer by making a crystal grow to a direction nearlyparallel to a surface of the intermediate layer, and forming a pair ofelectrodes on the optical absorption layer.(Supplementary note 10) The method for making a photodetector accordingto supplementary note 9 comprising, forming a lower cladding layer onthe substrate, forming a semiconductor layer on the lower claddinglayer, and forming the optical waveguide and the growth underlayer byetching the semiconductor layer, and preparing the first element.(Supplementary note 11) The method for making a photodetector accordingto supplementary note 9 or 10, wherein the optical absorption layer isone of IV-group and III-V-group semiconductor comprising one of asingle-crystal structure, a polycrystalline structure, and an amorphousstructure.(Supplementary note 12) The method for making a photodetector accordingto supplementary note 9 or 10, wherein one of IV-group and III-V-groupsemiconductor comprising one of a single-crystal structure, apolycrystalline structure, and an amorphous structure is used in thegrowth underlayer.(Supplementary note 13) The method for making a photodetector accordingto supplementary note 11, wherein the optical absorption layer has asingle crystal structure.(Supplementary note 14) The method for making a photodetector accordingto any one of supplementary notes 9, 10, 11, 12 and 13 comprising,removing the growth underlayer and the crystal formed on the growthunderlayer, after forming the optical absorption layer on theintermediate layer.(Supplementary note 15) The method for making a photodetector accordingto any one of supplementary notes 9, 10, 11, 12, 13 and 14, wherein thecrystal is formed using the chemical vapor deposition method.(Supplementary note 16) A method for making an optical communicationdevice, comprising forming an optical communication device using thephotodetector made by the method for making a photodetector according toany one of supplementary notes 9, 10, 11, 12, 13, 14 and 15.

FIELD OF INDUSTRIAL APPLICATION

The present invention is applied to a photodetector, an opticalcommunication device equipped with the same, a method for making thephotodetector, and a method for making the optical communication device.

DESCRIPTION OF THE CODES

-   100, 150, 200, 300, 400, 500, 600, 700 photodetector-   101, 201, 301, 401, 501 optical waveguide-   102, 202, 302, 502 optical absorption layer-   103, 203, 303, 503 electrodes-   104, 204, 304, 504 substrate-   105, 205, 305, 505 lower cladding layer-   106, 206, 306, 506 intermediate layer-   107, 207, 307, 507 upper cladding layer-   108 p-type semiconductor-   109 n-type semiconductor-   110 p-electrode-   111 n-electrode-   113, 213 growth underlayer-   114, 214 longitudinal growth semiconductor layer-   115 semiconductor layer-   316 optical signal-   417 taper structure-   518 Si_(y)Ge_(1-y) (0≦y≦1, y≠x) layer-   620 lens-   621 electric wiring-   622 pre-amplifier IC-   623 chip carrier-   624 optical fiber-   626 module case-   728 inter-chip connection module-   729 LSI package-   730 electric wiring via for modulation-   731 VCSEL light source-   732 electric wiring via for photodiode-   733 light source and electric wiring layer for modulation-   734 optical signal output fiber-   735 first mounting board-   736 concave mirror-   737 second mounting board-   738 electric wiring layer for photodiode-   739 optical signal input fiber

The invention claimed is:
 1. A photodetector comprising: a substrate; alower cladding layer arranged on the substrate; an optical waveguidearranged on the lower cladding layer; an intermediate layer arranged onthe optical waveguide; a optical absorption layer arranged on theintermediate layer; a pair of electrodes arranged on the opticalabsorption layer, and wherein the optical absorption layer comprises aIV-group or III-V-group single-crystal semiconductor, and the opticalabsorption layer absorbs an optical signal propagating through theoptical waveguide.
 2. The photodetector according to claim 1, whereinthe optical absorption layer comprises Si_(x)Ge_(1-x) (0≦x≦1).
 3. Thephotodetector according to claim 1, wherein only an edge portion of theoptical absorption layer is arranged on the optical waveguideinterleaved with the intermediate layer.
 4. The photodetector accordingto claim 1, wherein an edge portion of the optical waveguide has a taperstructure, and the edge portion is arranged under the optical absorptionlayer interleaved with the intermediate layer.
 5. An opticalcommunication device, comprising the photodetector according to claim 1.6. A method for making a photodetector comprising: making a crystal growon a growth underlayer in a first element which comprises a substrate, alower cladding layer arranged on the substrate, an optical waveguidearranged on the lower cladding layer, an intermediate layer arranged onthe optical waveguide, and the growth underlayer arranged on thesubstrate, to a direction nearly vertical to the growth underlayer;after the grown crystal having reached the height of the intermediatelayer, forming a optical absorption layer on the intermediate layer bymaking the crystal grow to a direction nearly parallel to a surface ofthe intermediate layer; and forming a pair of electrodes on the opticalabsorption layer.
 7. The method for making a photodetector according toclaim 6, further comprising: forming a lower cladding layer on thesubstrate; forming a semiconductor layer on the lower cladding layer;and forming the optical waveguide and the growth underlayer by etchingthe semiconductor layer, and preparing the first element.
 8. The methodfor a photodetector according to claim 6, wherein the optical absorptionlayer is one of IV-group and III-V-group semiconductor comprising one ofa single-crystal structure, a polycrystalline structure, and anamorphous structure.
 9. The method for making a photodetector accordingto claim 8, wherein the optical absorption layer has a single-crystalstructure.
 10. A method for making an optical communication device,comprising: forming an optical communication device using thephotodetector made by the method for making a photodetector according toclaim
 6. 11. The photodetector according to claim 2, wherein only anedge portion of the optical absorption layer is arranged on the opticalwaveguide interleaved with the intermediate layer.
 12. The photodetectoraccording to claim 2, wherein an edge portion of the optical waveguidehas a taper structure, and the edge portion is arranged under theoptical absorption layer interleaved with the intermediate layer. 13.The photodetector according to claim 3, wherein an edge portion of theoptical waveguide has a taper structure, and the edge portion isarranged under the optical absorption layer interleaved with theintermediate layer.
 14. An optical communication device, comprising thephotodetector according to claim
 2. 15. An optical communication device,comprising the photodetector according to claim
 3. 16. An opticalcommunication device, comprising the photodetector according to claim 4.17. The method for a photodetector according to claim 7, wherein theoptical absorption layer is one of IV-group and III-V-groupsemiconductor comprising one of a single-crystal structure, apolycrystalline structure, and an amorphous structure.
 18. A method formaking an optical communication device, comprising: forming an opticalcommunication device using the photodetector made by the method formaking a photodetector according to claim
 7. 19. A method for making anoptical communication device, comprising: forming an opticalcommunication device using the photodetector made by the method formaking a photodetector according to claim
 8. 20. A method for making anoptical communication device, comprising: forming an opticalcommunication device using the photodetector made by the method formaking a photodetector according to claim 9.